
361
42073A-MCU Wireless-02/13
ATmega2564/1284/644RFR2
22 SPI- Serial Peripheral Interface
22.1 Features
The Serial Peripheral Interface (SPI) allows high-speed synchronous data transfer
between the ATmega2564/1284/644RFR2 and peripheral devices or between several
AVR devices.
The ATmega2564/1284/644RFR2 SPI includes the following features:
Full-duplex, Three-wire Synchronous Data Transfer
Master or Slave Operation
LSB First or MSB First Data Transfer
Seven Programmable Bit Rates
End of Transmission Interrupt Flag
Write Collision Flag Protection
Wake-up from Idle Mode
Double Speed (CK/2) Master SPI Mode
22.2
Functional Description
195 must be written to zero to enable SPI module. The block diagram of the SPI
The interconnection between Master and Slave CPUs with SPI is shown i
n Figure 22-2on page 362. The system consists of two shift Registers, and a Master clock generator.
The SPI Master initiates the communication cycle when pulling low the Slave Select SS
__
pin of the desired Slave. Master and Slave prepare the data to be sent in their
respective shift Registers, and the Master generates the required clock pulses on the
SCK line to interchange data. Data is always shifted from Master to Slave on the Master
Out – Slave In, MOSI, line, and from Slave to Master on the Master In – Slave Out,
MISO, line. After each data packet, the Master will synchronize the Slave by pulling
high the Slave Select, SS
__
, line.
When configured as a Master, the SPI interface has no automatic control of the SS
__
line.
This must be handled by user software before communication can start. When this is
done, writing a byte to the SPI Data Register starts the SPI clock generator, and the
hardware shifts the eight bits into the Slave. After shifting one byte, the SPI clock
generator stops, setting the end of Transmission Flag (SPIF). If the SPI Interrupt Enable
bit (SPIE) in the SPCR Register is set, an interrupt is requested. The Master may
continue to shift the next byte by writing it into SPDR, or signal the end of packet by
pulling high the Slave Select, SS
__
line. The last incoming byte will be kept in the Buffer
Register for later use.
When configured as a Slave, the SPI interface will remain sleeping with MISO tri-stated
as long as the SS
__
pin is driven high. In this state, software may update the contents of
the SPI Data Register, SPDR, but the data will not be shifted out by incoming clock
pulses on the SCK pin until the SS pin is driven low. As one byte has been completely
shifted, the end of Transmission Flag, SPIF is set. If the SPI Interrupt Enable bit, SPIE,
in the SPCR Register is set, an interrupt is requested. The Slave may continue to place
new data to be sent into SPDR before reading the incoming data. The last incoming
byte will be kept in the Buffer Register for later use.